Method of treating stainless steel



.1. A. HENRlcKsoN Erm. 3,499,803

METHOD OF TREATING STAINLESS STEEL Filed Feb. 13, 1967 March 10, 1970 JOH/V A. HE/VR/CKSON' and NICHOLAS MAKR/DES By M Attorney United States Patent O U.S. Cl. 14S- 12.1 6 Claims ABSTRACT OF THE DISCLOSURE A method of treating non-austenitic stainless steel of initially low carbon content to increase the carbon level in a manner to preclude formation of large primary carbides. The carbon level is increased by carbon diffusion from carbon steel disposed as cladding on opposite surfaces of the stainless steel. After hot rolling to metallurgically bond the carbon cladding to the stainless steel core, the composite is heat treated to diffuse carbon from the carbon steel into the stainless core to increase the carbon content of it to above about 0.7%.

This invention relates to a method for producing high carbon stainless steel products which are useful for producing razor blades and cutlery. The product produced is characterized by the absence of large primary carbides which renders it particularly suitable for use as cutting edges.

Stainless steels have recently been used for the production of a significant portion of the razor blades marketed. The usual stainless steels which have been used for this purpose contain nominally 0.95% carbon, 0.9% manganese, 0.02% phosphorus, 0.02% sulfur, 0.25% silicon and 13.5% chromium. Razor blades, for example, of this composition may be used much longer than high-carbon, lowalloy steels formerly employed for this purpose. However, although stainless steels are superior to the carbon steels formerly used, a number of problems exist in razor blade production of stainless steel blades not only for the steelmaker but for the razor blade manufacturer as well. One major problem in the manufacture of razor blades is the presence of large carbides which may form during the solidifcation of molten stainless steel. These primary carbides are often the cause of tear outs which means that the carbides are pulled out in grinding or honing operations thereby seriously affecting the blades, particularly when the tear out appears at the cutting edge. As distin- V guished from secondary carbides, primary carbides are larger and will not dissolve during normal heat treating. If, however, the carbon content of the stainless steel is maintained at about 0.6% or lower, primary carbides may be avoided. This, however, is at the expense of hardness. Steels with low carbon do not exhibit satisfactory hardness for applications such as razor blades.

The present method avoids the aforementioned difficulties. It is applied to non-austenitic, stainless steels, that is, to ferritic or martensitic stainless steels, having a carbon content below that at which massive carbides will form. Austenitic stainless steels would not be used because of their inability to harden to required levels. The invention involves increasing the carbon content of the martensitic or ferritic stainless steel to the amount required to provide the desired hardness for such applications as razor blades by diffusing carbon into the stainless steel. The carbon content of the stainless steel may be provided in the desired range of 0.7 to 1.3% which is required for many cutlery applications. The preferred carbon content is, however, about 0.8 to 1% while 0.9% is preferred for razor blade production.

CII

Increasing the carbon content may be achieved by diffusion wherein the stainless steel is subjected at elevated temperatures to one of three carbon diffusion treatments. These treatments are:

(l) Diffusion by a carbon-containing gas; (2) Diffusion from a solid material; and (3) Carbon diffusion from a liquid source.

In the gas diffusion embodiment, gases which may be used include carbon monoxide, methane and ethane. In this embodiment, the composite structure in the form of hot or cold-rolled strip material is heated in an atmosphere of the aforementioned gases at temperatures within the range of about 1400 to 1900D F. for a time sufficient to develop the desired carbon content in the carbon steel portion of the composite, which usually takes between l to 24 hours.

A second method of carbon diffusion concerns the use of a carbon-containing liquid. Liquid baths of sodium cyanide, barium chloride and sodium carbonate may be used for this purpose. The required heating times and temperatures are the same as those used in gaseous diffusion.

The third method of carbon diffusion to increase the carbon level of ferritic or martensitic stainless steel is by the use of a carbon-containing solid. To obtain the desired carbon level in the stainless steel core, carbon steel having at least about 0.7% carbon is used. However, in an alternative embodiment, steel with a lower carbon content, but at least about 0.05% carbon, may be used if carburized to at least about 0.7% carbon.

Carbon diffusion can also be attained by intimate contact of the composite strip surface with solid carbon particles to carbonize the clad portion of the composite. The clad jacket in this case would be a low-carbon steel to facilitate initial processing. The subsequent treatment to obtain diffusion of the carbon to the stainless steel core would not change but the point in processing at which it may be applied can vary.

Example of a preferred embodiment which involves carbon diffusion by use of a solid, carbon-containing material in accordance with the invention is as follows:

Cold-rolled, annealed carbon-steel strips 12 inches long, 6 inches Wide and 0.04 inch thick, which contained about 1.25% carbon, were spot welded to a type 410 coldrolled, annealed stainless steel strip having the same dimensions and containing about 0.15% carbon. One carbon strip was spot welded to each face of the stainless steel insert. Six assemblies, each 0.120 :inch thick, were made in this manner and hot rolled at 2150 F. to about 0.090 inch thick composites to produce metallurgically bonded interfaces. The composites were then heated to 1450 F. in accordance with one embodiment of the invention at 150 F. per hour and then to 1600 F. at F. per hour in an air atmosphere. The composites were held at the latter temperture for four hours and then cooled to 1100 F. and finally air cooled to room temperature. The carbon content of the stainless steel insert after this treatment increased from 0.15 to 1.10% carbon. After a conventional hardening treatment, stainless steel with this carbon content developed a minimum hardness of 65 RC which is the specification requirement for stainless steel razor blade cutting edges.

A heat treatment at 1600" F. will increase the carbon level above the 0.70% minimum normally desired and will also spheroidize the carbides present. Thus, in diffusing at about this temperature, the primary carbides that heretofore caused tear outs do not develop.

A comparison of the metallurgical structure showing the presence of large primary carbides with conventionally produced stainless steel specimens and tl'ie product produced in accordance with the invention appear in FIG- URES 1 and 2, respectively. FIGURE 1 is a photomicrograph of a conventional stainless razor blade steel annealed at 1650 F., etched with HCl-picric etchant and at 1000 showing large primary carbides. The photomicrograph in FIGURE 2 is of a carburized steel core of stainless razor blade steel austenitized at 1950 F., quenched in water, requenched at 100 F. and ternpered at 400 F., treated with the same etchant and at the same magnification. As can be seen in FIGURE 2, the product produced in accordance with the invention contains only small secondary carbides which is not the case with stainless steel shown in FIGURE 1. Moreover, the structure shown in FIGURE 2 developed a hardness on the order of 65 RC after the hardening and tempering treatment described and thus meets the requirement for stainless steel razor blade cutting edges.

After spheroidize-annealing as discussed above, the assembly was cold rolled from about 0.09-inch thick strip to 0.0l-inch thick razor blade strip using four anneals with total reductions of about 40% between each anneal. The annealing cycles were conducted at 1300 F. The carbon content of the carbon steel after diffusion is reduced from about 1.25% to about 0.20%. This steel core can be used in the manufacture of a clad stainless Steel razor blade or in the production of an all stainless razor blade by pickling or scaling to remove the carbon steel jacket.

We have found that ditfusion of carbon from a solid material to the stainless steel may be obtained at temperatures within the range of 1400 to 2400 F. However, treatment at about 1650 F. or less is necessary because of the simultaneous action of diffusion and spheroidization of the carbides. For diffusion to occur when employing a carbon steel, it is necessary that carbon be in solid solution. In this case, the temperature must be high enough so that the carbon steel will be in the gamma temperature range which begins at about 1400 F. The gamma phase is the carbon rich phase and dissolves many times the amount of carbon that the alpha or ferrite phase dissolves. Although carbon will diffuse more rapidly at high annealing temperatures, temperatures below about 1650 F. are additionally necessary to avoid the severe grain growth that occurs at higher temperatures not only in the stainless steel but also in the carbon steel if this is used as a source of carbon.

As mentioned previously, it is also possible to use steel with as little as about 0.05% carbon as cladding for the stainless core. In this embodiment, the steel cladding, after hot rolling to bond the core and cladding, is carburized in any suitable or conventional manner to raise the carbon content of the cladding steel to at least about 0.7%. The carburizing treatment described above in connection with gaseous diifusion may be used here.

As described above, the elimination of primary carbides by practicing the present invention minimizes or eliminates the occurrence of tear outs during the grinding of blade cutting edges. In addition, this improves the life of the blade-slitting and perforating dies and increases the steel hardening rates by virtue of the faster solution rate of fine carbide compared with that of larger carbides.

It is apparent from the above that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims wherein what is claimed is:

1. A method of treating stainless steel to produce a product which is characterized by the absence of large primary carbides and is useful for manufacturing cutlery, and wherein the carbon content of the ultimate stainless product is greater than the initial carbon content of the stainless steel which method comprises assembling a sandwich of a non-austenitic stainless steel core within carbon steel claddings, the carbon content of said stainless steel being initially less than about 0.6% and the carbon content of said carbon steel being initially at least about 0.7%, hot rolling said sandwich to metallurgically bond the core and claddings into a composite, and holding the hot-rolled composite at about 1400 to 1650o F. to diffuse carbon from said carbon steel into the stainless steel core so as to increase the carbon content thereof to above about 0.7% and lower the carbon content of said claddings to about 0.2%.

2. A method as in claim 1 wherein the carbon content of the stainless steel core is increased to Within the range of 0.7 to 1.3%.

3. A method as in claim 2 wherein the carbon content of the stainless steel core is increased to within the range of 0.8 to 1.0%.

`4. A method of treating steel comprising assembling a sandwich of a non-austenitic stainless steel core within carbon steel claddings, the carbon content of said stainless ysteel being initially less than about 0.6% and the carbon content of said carbon steel being initially at least about 0.05 hot rolling said sandwich to metallurgically bond the core and claddings into a composite, carburizing the composite to raise the carbon content of said carbon steel to at least 0.7%, and holding the hot-rolled composite at about 1400 to 1650L7 F. to diffuse carbon from said carbon steel into the stainless steel core so as to increase the carbon content thereof to above about 0.7% and lower the carbon content of said cladding to about 0.2%.

5. A method as in claim 4 wherein the carbon content of the stainless steel core is increased to within the range of 0.7 to 1.3%.

6. A method as in claim 5 wherein the carbon content of the stainless steel core is increased to within the range of 0.8 to 1.0%.

References Cited UNITED STATES PATENTS 2,531,731 11/1950 Hibbert 14S- 12.1 X 3,116,180 12/1963 Malzacher 148-135 X 3,313,660 4/1967 Vordahl 148-127 X CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT oFFICE CERTIFICATE 0F CORRECTION Patent No. 3,499 ,803 March l0 1970 John A. Henrckson et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 46, the claim reference numeral "5" should Signed and sealed this 15th day of December 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

